The technique disclosed in the present specification relates to a communication apparatus for performing a noncontact communication, and more particularly, to a communication apparatus for performing communication by proximity wireless transfer using a weak UWB signal.
The noncontact communication is radio technology for transferring data in a transmission distance of zero to about several dozen centimeters, and has an advantage in that it is less affected by interference because there are less reflected waves from reflecting objects existing therearound and that it is not necessary to consider ensuring confidentiality and preventing hacking in a transmission path. Noncontact communication methods include a capacitive coupling method, an electromagnetic induction method, and a radio communication method.
Noncontact communication techniques that can be applied to high-speed communication include a proximity wireless transfer technique such as “TransferJet (registered trademark)” (for example, see www.transferjet.org/en/index.html (as of Jun. 14, 2011)). This proximity wireless transfer technique is basically a method for transmitting a signal using coupling action of induction field, and a communication apparatus thereof includes a communication circuit unit configured to process a high-frequency signal, a coupling electrode spaced apart and arranged at a certain height with respect to a ground, and a resonant unit efficiently providing the high-frequency signal to the coupling electrode. The coupling electrode or a component including the coupling electrode and the resonant unit will also be referred to as “high-frequency coupling device” or “coupler” in this specification.
One of the features of the proximity wireless transfer technique includes achievement of ultra high-speed data transmission at a speed of about 100 Mbps using a weak UWB (Ultra Wide Band) signal. Another feature of the proximity wireless transfer technique is the UWB communication, of which transmitting power is low. Therefore, an electric field strength (strength of radio wave) at a distance of three meters from radio equipment is equal to or less than a predetermined level. In other words, it is a weak radio which is almost at a noise level for another radio system existing nearby. For this reason, users can use this without obtaining any license (for example, see Radio Law Enforcement Regulations (Radio Regulatory Commission Rules No. 14, 1950), Section 6(1)(i)).
On the other hand, many information devices such as a personal computer (PC), a digital camera, a portable telephone, and a PDA (Personal Digital Assistant) have a slot into which a cartridge-type external memory such as a memory stick (registered trademark) and an SD (Secure Digital) card is inserted, thus achieving expansion of the storage capacity. Furthermore, memory cards having a radio communication function therein have been suggested (for example, see Japanese Patent Application Laid-Open No. 2001-77878 and Japanese Patent Application Laid-Open No. 2006-216011). Even if an information device such as a digital camera does not have a communication interface in itself, the information device can wirelessly transmit, in a direct and easy manner, large-capacity data such as still pictures and motion pictures recorded in an internal memory or a memory card when the memory card having a radio communication function is attached to a slot (for example, see www.eyefi.co.jp (as of Jun. 14, 2011)).
Alternatively, the proximity wireless transfer technique may be provided in a memory card (for example, see Japanese Patent Application Laid-Open No. 2010-67060). When the proximity wireless transfer technique is provided in a memory card, a high-frequency coupling device can be easily made by forming a ground and a coupling electrode as a wiring pattern on the same memory card substrate (for example, see Japanese Patent No. 4345891, FIG. 23).
In this case, when the memory card is inserted into the slot of the information device made of a metallic casing, there is a possibility that sufficient communication performance may not be achieved with the proximity wireless transfer technique using a low transmitting power. In particular, digital cameras are often made of metallic casing because the digital cameras has to have strength that can withstand drop even though the digital cameras are small and thin.
For this reason, when the proximity wireless transfer technique is provided in the memory card, it is preferable to improve the transmitting power, i.e., the coupling action of the induction field, at an end surface portion of the memory card, which is exposed to the outside when the memory card is inserted into the slot of the information device. However, as the induction electric field strength is increased, the strength of the radiation electromagnetic field increases, which is likely to affect another radio system existing nearby, and this makes it necessary to obtain a license to use the memory card. Moreover, this causes radiation of radiation electromagnetic field that is not used in the proximity wireless transfer, and the electric power is consumed. This weakens the induction electric field strength in a near field that is originally necessary for the proximity wireless transfer.
When the memory card is inserted into the inside of the metallic casing, the radiation electromagnetic field is shielded by the casing and is not radiated to the outside. Therefore, there may be a method for obtaining authentication under the Radio Law for the digital camera having the memory card inserted therein. For example, the design of the memory card having the proximity wireless transfer technique can be optimized and specialized for a particular type of digital camera. However, a vendor manufacturing and selling memory cards having the proximity wireless transfer technique is required to obtain authentication under the Radio Law for each type of digital camera instead of obtaining authentication under the Radio Law for the memory card itself, or the vendor is required to develop and manufacture memory cards for each type of digital camera, and this burden would be the problem. In other words, it is preferable for the memory card itself to satisfy the regulations under the Radio Law and ensure sufficient performance of the proximity wireless transfer even when the memory card is inserted into the metallic device casing.